In recent years, because of the fast development in electronic technology portable computers, more particularly lap-top type or note book type computers become more and more popular. The main advantage of portable computers is that they can be conveniently carried with oneself for use at anytime and anywhere. However, in designing a portable computer, one may encounter two problems. The first problem is that the battery which provides a computer with necessary working voltage occupies space and increases the total weight of a computer. When AC power or car power source is utilized for a portable computer, the battery of such a portable computer becomes useless and somewhat a burden. The other problem in designing a portable computer is the limitation of the available space for expansion cards. Unlike regular desktop computers which generally comprise several expansion slots for external expansion. In case to provide more expansion slots, the size of a portable computer will become more heavy and its acceptability to the consumers will be reduced. Therefore, regular portable computers generally do not have expansion slots for external connection.